Large structures such as gravity bases for fixed platforms or for tension leg platforms have to be lowered to the sea bed in deep waters. The problem is how to provide control for the structure, as the structure goes below the surface of the water when lowered. Due to the size of such structures, their added mass is commonly very large, making them very inert. Control of such structures by the use of cranes which are connected rigidly to the structure can therefore lead to excessively high crane hook motions in the event of wave induced motions of the crane vessel.